Photographing Star Trails

Project for Elementary Astronomy

You've seen beautiful long-exposure photographs of star trails, vividly illustrating
earth's daily rotation relative to the stars. This project offers you an opportunity
to make your own star trail photos.

For this project you need suitable equipment:

Camera. You can use either a digital SLR camera or a 35mm film camera, provided that it is
capable of taking long time exposures. Most "point-and-shoot" cameras cannot take long time
exposures. If you are in doubt about the suitability of your camera or
how to use it, bring it to your instructor.

Tripod. A tripod allows you to point the camera and hold it steady
for long exposures. In principle you could rest the camera on a large sack of beans on a
sturdy table, but it's really best to have a good tripod.

Film (if using a film camera). Ordinary color print film is best, preferably with an
ISO rating of about 200. Slower film (ISO 100 or so) is fine if you have a reasonably
fast lens (f2 or less). If you use ISO 400 film with such a lens,
compensate by stopping down the lens somewhat. Avoid film speeds higher than ISO 400.
You can also use color slide film if you have a way of making prints from it (perhaps
with your own scanner).

Locking cable release or the equivalent. You need a way of keeping
the camera's shutter open without holding your finger on the button continuously. Some
cameras have a built-in shutter lock that works great, but most require an add-on
cable release or switch which you can lock.

Timer or watch. You'll need a way of timing the length of your
exposures (typically 10 minutes to one hour or more). A timer that you set to beep after
a certain amount of time is most convenient, but an ordinary wristwatch will do. A digital
camera may have a built-in timer.

Pencil, paper, and flashlight for making notes.

To conduct this observing project, follow this procedure:

Start this project well before the due date, in case your
photos don't turn out the first time.

Test your equipment. While indoors, practice attaching
the camera to the tripod and operating the shutter and cable release to take long
exposures. If you're using a film camera, do this before you put the film in.

If you're using a film camera, put the film in and take a couple
of ordinary short-exposure daytime photos to start the roll. (This will enable whoever
processes the film to tell where one frame ends and the next begins.)

If you're using a digital camera, try some 10-minute test
exposures at night, to make sure they turn out. Figure out which aperture and ISO
settings give the best results. Determine how many 10-minute exposures you can
take before your battery becomes discharged, and plan accordingly to use extra
batteries or to recharge between photos.

Make your photos from a dark location with a clear view
of the sky, away from any direct light sources. A suburban back yard might work
if there aren't too many trees or streetlights. A local park or undeveloped area may be a better option.
It isn't necessary to get completely away from civilization, but you mustn't be
near any brightly lit areas either. Darker is better.

Make your photos on a dark night when the sky is clear and the
moon isn't near full. A little moonlight is acceptable as long as it isn't too bright.

Set your lens to focus at infinity. Some lenses are conveniently marked for focusing.
With others you'll have
to point at a bright far-away object and focus by eye.

For each photo you take, be sure to write down the time when
you started the exposure, the time when you ended, your camera settings, the direction
the camera was facing, and any other
observations such as sky conditions or planes that flew through the field of view.

Make your first photos with the camera pointed east. Tilt
the camera upward so the sky fills most of the frame, but with the horizon or a tree
or some other terrestrial object visible near the bottom. Take a 10-minute exposure with the lens
opened to its widest aperture. Then close the lens one or two stops and take another
10-minute exposure, then close the lens another one or two stops and take a third
10-minute exposure. Try some longer exposures if you wish, preferably with the lens
stopped down somewhat. If you're using a digital camera, use whatever aperture and ISO settings
give a reasonably dark sky while still showing plenty of stars.

Repeat the same procedure for photos pointing south and west.

Now point your camera to the north, so the north star is visible
in the photo. Take a half-hour exposure with the aperture 2 or 3 stops below its widest
opening. Try some other aperture settings or longer exposures if you have time.

Check your camera lens for dew after each exposure. Under humid
conditions, dew may form on the lens surface, blurring your photos. If this happens, you'll
need to warm up the lens indoors and perhaps rig up a lens hood to prevent dew from forming.

Print the photos in whatever way you can, either as traditional photographic prints, or
digitally placed into the pages of your report. If you are using film, be sure to tell the
people who process the film that these are night photos that are supposed to be dark. Otherwise
their equipment will automatically try to lighten the prints, ruining them in the process.
If they don't know what you're talking about, take your film elsewhere for processing.

For your report, analyze and discuss your photos as follows:

For each of the four directions (east, south, west, and north),
choose the best of your photos. Be sure to attach these to your report, labeling each with
the direction, time, and camera settings. In your report, discuss which camera settings gave
the best results and why.

Discuss the direction in which the stars appear
to move in each of your photos. Explain these directions in terms of the earth's rotation
and the latitude of your location.

Pick one of your photos and use it to determine the rate at which
the stars appear to rotate around us, in degrees per hour. You can do this in either of
two ways. Your east- and west-facing photos show stars rotating in circles with you at
the approximate center. By comparing one of these photos to a good star chart and identifying
the stars that are shown, you can determine the scale of the photo in millimeters (or inches)
per degree. Once you know the scale, measure the length of any star trail to determine
how many degrees of rotation occurred during the photo. Then calculate the number of
degrees of rotation per hour. Alternatively, you can use your north-facing photo in which
the center of the circles is approximately at the north star. There's no need to determine
a scale on this photo, but you do need a protractor to carefully measure angles. Either way,
explain your method and show all your calculations in your report. Also compare your result
to what you would expect, based on what you have learned in this course.

Find a star in one of your photos that is noticeably yellow or orange
in color, and one that is noticeably bluish. Identify each of these stars by comparing your
photos to a good star chart, and look up the spectral type of each star and the corresponding
temperature. Discuss in general how you can determine the temperature of a star from its color.